Devices and methods for widening tube shaped organs, in general and in the human body, in particular, are known in the art.
U.S. Pat. No. 5,716,410, to Wang et al, is directed to a catheter for vascular use. The catheter includes a directing mechanism and an inflatable balloon at the tip. The tip also includes a temporary stent, made of thermoplastic material. The catheter is inserted into the body of the patient, via a wide surface artery and the operator directs the catheter toward the destination location. When the tip is positioned at the destination location, then the operator pumps fluid to the balloon, via a tube, running along the catheter and starts inflating the balloon.
Reference is now made to FIG. 2, provided by Wang et al as FIG. 1, which is an illustration of a catheter, known in the art.
At the same time, the thermoplastic stent is heated, thereby unlocking its shape. The balloon, as it inflates, applies circumferencial pressure on the stent and the tubular organ, thereby forcing them to extend. Accordingly, the circumference of the tubular organ becomes larger. When the balloon brings the thermoplastic stent to the destination diameter, then the thermoplastic stent is cooled, thereby fixing its shape at a larger diameter, where it supports the tubular organ at an enlarged position.
Then, the operator, reduces the pressure within the balloon, which in turn deflates and becomes smaller than the enlarged circumference of the tubular organ. The thermoplastic stent is then kept within the tubular organ for a time period which may range from a few minutes to as long as a week. Finally, the thermoplastic stent is reheated, thereby unlocking its shape, and enabling its removal from the body of the patient.
It will be appreciated by those skilled in the art, that a balloon is often longer than the segment to be treated. Hence, the inflatable portion of the balloon extends beyond the desired segments to healthy segments, and can cause damage thereto.
Furthermore, when a balloon is inflated inside a blood vessel, it occludes the blood flow distally and becomes a full barrier for any blood flow therein. It will be appreciated by those skilled in the art that many blood vessel related balloon treatments are performed in coronary arteries. Accordingly, such a procedure, while blocking blood flow through the treated blood vessel, may cause ischemia or even cardiac arrest.
The balloon blocks the blood flow both in the axial (through the blood vessel) and radial directions blocking branches.
The complete obstruction, a balloon related treatment is usually limited to one or two minutes of inflation, since the patient can not tolerate long time inflations, because of severe pain and chest discomfort, due to ischemia.
It will be appreciated by those skilled in the art that a blood vessel is generally a flexible organ. This fact, combined with the short time period in which a balloon expands the circumference of the blood vessel, (as much as 40% of the cases) causes the balloon treated blood vessel to assume its original size (recoil) immediately, or within few months shortly after the treatment.
A stent is generally an element which is inserted into the tubular organ with the aid of a catheter. The initial shape of a stent is of an elongated cylinder, having a diameter which is smaller than the narrowed section of the tubular organ, through which it has to pass. At the beginning of the treatment, the stent is positioned in the stenosed segment.
Then, the circumference of the stent is widened, by various methods, known in the art. One of these methods includes inserting a balloon into the stent and applying pressure by inflating it therewith. Accordingly, the stent widens, thereby applying pressure on the narrowed blood vessel. As a result, the stent widens the cross section of the diseased segment.
Finally, the balloon is deflated and is removed from the stent, which remains in its widened position, forcing the widened blood vessel to remain at its new state. Afterwards, the stent is covered by local tissue and is anchored thereto. This poses a disadvantage in the usage of a stent since such a stent can not be removed. A stent is an alien element within a living organism, which might produce thrombus in it.
In many cases (15%-40%) instent stenosis occurs. The mechanism of this narrowing is intimal proliferation thereby causing a new blockage at the same location. It is known by those skilled in the art that sometimes, such a reoccurring blockage is difficult to treat and in some cases, surgery is needed, to remove and replace the clogged section.
In some instances the stent may be lost and migrate distally in the coronary artery, or sometimes in the aorta and its branches. The stent also can be stucked.
Shape memory effect (SME) is a phenomena, in shape memory alloys (SMA) of a reversible transition from one solid phase into the other (i.e., from Martensite into Austenite or from Austenite to Martensite). Heating the alloy causes the transition from Martensite into Austenite. Cooling the alloy causes the reverse transition, from Austenite into Martensite. NiTi alloys are examples for such shape memory alloys.
Martensite and Austenite are two solid state phases, which are typical for alloys. Each of these phases is characterized in a certain crystaline structure.
Basically there are two types of shape memory effects. The first type is called the one way shape memory effect (OWSM), where the material transits from one of the above phase states to the other, only once. The second type is called the two way shape memory effect (TWSM), where the material transits from one of the phase states to the other and back in a reversible process.
Shape memory alloys, such as Ni--Ti, Ni--Ti--X, Cu--Ni--Al, Cu--Zn--Al, Fe--Mn--Si, Ni--Ti--Co, Ni--Cu--X, Ni--Al and the like, are known in the art. These alloys exhibit a shape memory effect. In the martensite condition, the shape alloy material is relatively flexible and soft, and can be easily deformed. When the material undergoes the transition into an austenite state, it becomes more rigid, and is able to apply force and generate work, deform and enlarge the cross section of the blood vessel.
U.S. Pat. No. 5,540,713 to Shnepp-Pesch et al, is directed to an apparatus for widening a stenosis in a body cavity, also known as a shape memory stent. Shnepp-Pesch describes a stent made from a shape memory alloy, assuming a first predetermined shape at a first predetermined temperature and second predetermined shape at a second predetermined temperature. When heated from the first temperature to the second one, the shape memory stent changes its shape from a narrow generally cylindrical shape to a wider generally cylindrical shape.
Shnepp-Pesch describe a plurality of shapes which are applicable as shape memory stents. Reference is now made to FIGS. 1A-1H, provided by Shnepp-Pesch et al, as FIGS. 1A, 1B, 2A, 2B, 3A, 3B, 4A and 4B, respectively. These figures describe four shape memory stent structures, each at two states, one narrowed and the other enlarged.
It will be appreciated by those skilled in the art that this stent basically suffers the same disadvantages as any other stent, known in the art, as listed above.